Locking coupler device for spine alignment

ABSTRACT

Embodiments are directed to an apparatus for aligning vertebrae of a curved spine that may include a coupler having a longitudinal axis, a proximal end, and a distal end, wherein the proximal end comprises first threads, wherein the distal end comprises a mating surface for disposition around a rod attached to a spinal column, and wherein the coupler applies a compressive clamping force to the rod. The apparatus further may include a nut having second threads corresponding to the first threads, wherein the nut is attachable to the coupler proximal end. The apparatus further may include a driver disposable at least partially around the coupler, wherein the driver applies a clamping force to the coupler, and wherein the nut applies a linear force to the driver when tightened.

BACKGROUND

A common medical problem is a curved spine caused by dislocatedvertebra. A curved spine can cause pain, suffering, restricted motion,and a loss of quality of life for a person. A curved spine can be causedby a congenital defect, an injury or accident, or by gradual decay ofsupporting bone structure over time. In some cases, alleviating thesymptoms of a curved spine does not require surgery. However, surgery isoften required to correct the alignment of vertebrae and to relieve thepain. Surgery may involve fusing two adjacent vertebrae together orother invasive procedures. However, patients with fused spines oftenexperience limited motion and, in some cases, a stooped appearance. Whatis needed are improved apparatus to counteract the curved spine in aneffort to reduce the effects and to relieve a patient's limited motion,pain, and suffering.

SUMMARY

In an exemplary embodiment, the present disclosure provides an apparatusfor aligning vertebrae of a curved spine. The apparatus may include acoupler having a longitudinal axis, a proximal end, and a distal end,wherein the proximal end comprises first threads, wherein the distal endcomprises a mating surface for disposition around a rod attached to aspinal column, and wherein the coupler applies a compressive clampingforce to the rod. The apparatus may further include a nut having secondthreads corresponding to the first threads, wherein the nut isattachable to the coupler proximal end. The apparatus may furtherinclude a driver disposable at least partially around the coupler,wherein the driver applies a clamping force to the coupler, and whereinthe nut applies a linear force to the driver when tightened.

In another exemplary embodiment, the present disclosure provides asystem for aligning vertebrae of a curved spine. The system may includea pair of locking coupler assemblies. Each of the locking couplerassemblies may include a coupler having a longitudinal axis, a proximalend, and a distal end, wherein the proximal end comprises first threads,wherein the distal end comprises a mating surface for disposition arounda rod attached to a spinal column, and wherein the coupler applies acompressive clamping force to the rod. Each of the locking couplerassemblies may further include a nut having second threads correspondingto the first threads, wherein the nut is attachable to the couplerproximal end. Each of the locking coupler assemblies may further includea driver disposable at least partially around the coupler, wherein thedriver applies a clamping force to the coupler, and wherein the nutapplies a linear force to the driver when tightened. The system mayfurther include a distractor tool attachable to the locking couplerassemblies and operable to move apart the locking coupler assembliesfrom one another.

In yet another embodiment, the present disclosure provides a method foraligning vertebrae of a curved spine. The method may include tighteninga first locking coupler assembly onto a rod, wherein the rod is attachedto a spinal column by way of bone fasteners distributed along the spinalcolumn. The method may further include tightening a second lockingcoupler assembly onto the rod. The method may further include forcingthe first locking coupler assembly to move along the rod way from thesecond locking coupler assembly such that one or more of the bonefasteners are forced more into alignment with an adjacent one of thebone fasteners.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some examples of thepresent invention, and should not be used to limit or define theinvention, wherein:

FIG. 1 illustrates components of a locking coupler assembly.

FIG. 2 illustrates a locking coupler assembly partially assembled.

FIG. 3 illustrates a locking coupler assembly in an untightened state.

FIG. 4 illustrates a cutaway view of a locking coupler assembly in anuntightened state.

FIG. 5 illustrates a locking coupler assembly in a tightened state.

FIG. 6 illustrates a cutaway view of a locking coupler assembly in atightened state.

FIG. 7 illustrates a locking coupler assembly having a smooth matingsurface.

FIG. 8 illustrates a locking coupler assembly having a textured matingsurface.

FIGS. 9 and 10 illustrate a distractor tool, a rod, and two lockingcoupler assemblies in unlocked and locked states.

FIGS. 11 and 12 illustrate a rod affixed to vertebrae with two lockingcoupler assemblies being aligned by a distractor tool.

FIGS. 13 and 14 illustrate the rod affixed to vertebrae of FIGS. 11 and12 with two locking coupler assemblies being aligned by a distractortool at a different level.

DETAILED DESCRIPTION

Embodiments are directed to an apparatus for aligning vertebrae of acurved spine. More particularly, embodiments may include a lockingcoupler assembly that may be used in aligning vertebrae of a coveredspine. In operation, a pair of locking coupler assemblies may beattached a rod at adjacent segments. In at least one embodiment, the rodmay be placed but not fixed to a pedicle screw. The locking couplerassemblies may then be moved apart with at least one of the lockingcoupler assemblies moving along the rod toward the adjacent segment.This should place the adjacent segments more in line. After which, therod may be fixed to the pedicle screw, and the process may be repeatedat one or more additional levels in the vertebral column.

Embodiments of the locking coupler assembly may include a coupler, adriver, and a nut. In some embodiments, the coupler may have alongitudinal axis, a proximal end, and a distal end, where the proximalend includes first threads, the distal end has a mating surface fordisposition around a rod fixably attached to a vertebra, and the couplerapplies a compressive clamping force to the rod. In some embodiments,the nut may include second threads corresponding to the first threads ofthe coupler, where the nut is attachable to the proximal end of thecoupler. In some embodiments, the driver may be disposable at leastpartially around the coupler, where the driver can apply a clampingforce to the coupler, and where the nut applies a linear force to thedriver when tightened.

FIG. 1 illustrates components of a locking coupler assembly 100 inaccordance with example embodiments. As illustrated, the locking couplerassembly may include a coupler 102 with a proximate end 108 and a distalend 110. According to some embodiments, the coupler 102 may be a hollowcylinder-shaped structure with a set of first threads 112 at theproximate end 108 and a mating surface 114 at the distal end 110. In theillustrated embodiment, the distal end 110 also includes a split seam116, which may extend along a longitudinal axis of the coupler 102. Thesplit seam 116 may extend a distance from the distal end 110 towards theproximate end 108 and may define a moveable and spring-like extensionarm 118. The extension arm 118 may be configured to flex radiallytowards the longitudinal axis of the coupler 102. According to someembodiments, the extension arm 118 may include portions of the matingsurface 114. In some embodiments, the mating surface 114 may have asmooth surface (e.g., smooth finish 700 on FIG. 7) while in otherembodiments, the mating surface 114 is made of a textures or serratedsurface (e.g., textured finish 800 on FIG. 8) to aid in gripping a rodor other object positioned within the distal end 110 of the coupler 102.In some embodiments, the coupler 102 also may include an angled shoulder120 that can be located at or near a central region of the coupler 102.The angled shoulder 120 may be positioned along the coupler 102 so thatthe split seam 116 extends a distance further towards the proximate end108 than the angled shoulder 120. In some embodiments, the angledshoulder 120 may be positioned along the coupler 102 such that portionsof the angled shoulder 120 are located on the coupler 102 itself andform a portion of the extension arm 118, thus providing a location whereforce can be applied to the coupler 102 to make the extension arm 118flex relative to the coupler 102. In operation, when a longitudinalforce is applied to the angled shoulder 120, the extension arm 118 ispushed inwards towards the center or longitudinal axis of the coupler102 and compresses the mating surface 114 to form a clamping effectaround a rod, in accordance with example embodiments. According to someembodiments, the locking coupler assembly 100 is included of at leastone of stainless steel, titanium, nitinol, nylon, phenolic, resin, orcomposite material.

At least one embodiment of the locking coupler assembly 100 alsoincludes a driver 104. In some embodiments, the driver 104 may be madeof a hollow cylinder-like structure having a forcing surface 122 and anattachment lug 124. The forcing surface 122 may be angled to correspondto the angled shoulder 120 of the coupler 102. According to someembodiments, the forcing surface 122 has approximately the same angle asthe angled shoulder 120. In other embodiments, the forcing surface 122is orthogonal to the angled shoulder 120. In accordance with exampleembodiments, the driver 102 is configured to slide over the proximateend 108 of the coupler 102 and over the first threads 112 of the coupler102 to allow the forcing surface 122 to couple with the angled shoulder120 of the coupler 102. By way of example, when the driver 104 and thusthe forcing surface 122, is driven towards the distal end 110 of thecoupler 102, the forcing surface 122 should apply a longitudinal forceto the angled shoulder 120 of the coupler 102, for example, to compressthe extension arm 118 inwards. By way of further example, the resultantmovement of the extension arm 118 should tighten the coupler 102 againstan object, such as a rod, positioned in proximity to the mating surface114. The attachment lug 124 may be configured to engage with aninstallation tool (e.g., distractor tool 900 on FIG. 9). The driver 104may also include a loading surface 128 that can be configured to receivea longitudinal force and transfer the force to the forcing surface 122.

At least one embodiment of the locking coupler assembly 100 alsoincludes a nut 106. In some embodiments, the nut 106 and the driver 104may be separate components. In other embodiments (not shown), the nut106 and the driver 104 may be integrally formed. As illustrated, the nut106 may have a set of second threads 126 that correspond to the set offirst threads 112 of the coupler 102. According to some embodiments, thesecond threads are positioned on an interior surface of the nut 106corresponding to the first threads 112 positioned on an exterior surfaceof the coupler 102. In other embodiments, the first threads 122 areinternal threads and the second threads 126 are external threads. Inoperation, for example, when the nut 106 is tightened against thecoupler 102 using the second threads 126 engaging the first threads 112,the nut 106 should apply a longitudinal force to the driver 104 byapplying force from the driving surface 130 of the nut 106 to theloading surface 128 of the driver 104. The driver 104 then should applya longitudinal force via the forcing surface 122 to the angled shoulder120 of the coupler 102. By way of example, applying the longitudinalforce to the angled shoulder 120 should compress the extension arm 118and collapses the split seam 116, thus tightening the coupler's 102distal end 110 against a rod or other object located in proximity to themating surface 114.

FIG. 2 illustrates a locking coupler assembly 100 partially assembled,in accordance with example embodiments, with the driver 104 positionedover the proximate end 108 of the coupler 102 and the forcing surface122 of the driver 104 in proximity to the angled shoulder 120 of thecoupler 102. The nut 106 is shown a distance away from the lockingcoupler assembly 100.

FIG. 3 illustrates a locking coupler assembly 100 in an untightenedstate, in accordance with example embodiments. As illustrated, the nut106 is loosely threaded to the coupler 102 and the driver 104 is notunder force. In this position, the extension arm 118 is not driveninwards by force, and an object in proximity to the mating surface 114is not fixably attached to the locking coupler assembly 100. In theunlocked state, the locking coupler assembly 100 can be positioned alongthe rod and placed in a location and orientation prior to tightening thenut 106 to form a fixed alignment.

FIG. 4 illustrates a cutaway view of a locking coupler assembly 100 inan untightened state, in accordance with example embodiments. Asillustrated, the forcing surface 122 of the driver 104 is not in contactwith the angled shoulder 120 of the coupler 102, thus no force isapplied to the extension arm 118. Also shown is the arrangement of thefirst threads 112 of the coupler 102 with the second threads 126 of thenut 106, for example, when the nut 106 is partially threaded onto theproximate end 108 of the coupler 102. Also shown is the arrangement ofthe driving surface 130 of the nut 106 in contact with the loadingsurface 128 of the driver 104. In this arrangement, as the nut 106 isturned, additional force is applied to the driver 104 through themovement of the driving surface 130 pushing on the loading surface 128.This causes the forcing surface 122 of the driver to move towards theangled shoulder 120 of the coupler 102.

FIG. 5 illustrates a locking coupler assembly 100 in a tightened statein accordance with example embodiments. As illustrated, the nut 106 maybe tightened against the coupler 102 and forcing the driver 104 tocompress the extension arm 118. According to some embodiments, the nut106 may be rotated, for example, in the direction shown by arrow 500 totighten the nut 106 onto the coupler 102. In some embodiments, thedirection may be clockwise. In other embodiments (not shown), thedirection may be counterclockwise. The nut 106 may be loosen whenrotated in the opposite direction. As the nut 106 is rotated in thedirection 500 and tightens onto the coupler 102, in accordance withexample embodiments, the nut 106 should apply a force in the directionshown by arrow 502 to the driver 104 that drives the driver 104 down thecoupler 102 compressing the extension arm 118.

FIG. 6 illustrates a cutaway view of a locking coupler assembly 100 in atightened state, in accordance with example embodiments. As illustrated,the forcing surface 122 of the driver 104 may be in full contact withthe angled shoulder 120 of the coupler 102. Thus positioned, theextension arm 118 may be pushed towards the longitudinal axis of thecoupler 102 and compresses an object, such as a rod, that is locatedadjacent to the mating surface 114.

Operation of locking coupler assembly 100 will now be described withrespect to FIGS. 3-6, in accordance with example embodiments. Inoperation, the coupler 102 of the locking coupler assembly 100 may beplaced in position around an object (not shown), such as a rod, with thelocking coupler assembly 100 in an untightened state, as shown on FIGS.3 and 4. In the untightened state, the locking coupler assembly 100 isnot tightened and the rod can be inserted adjacent to the mating surface114 without difficulty. Once the rod is in position, the nut 106 may berotated, as shown on FIGS. 5 and 6, and tightened to engage the driver104. As the nut 106 is tightened, the driver 104 should be forced todrive against the angled shoulder 120 of the coupler 102 and thus beginsto force the extension arm 118 inwards towards the rod. Additionaltightening of the nut 106 should increases the force to the driver 104and the extension arm 118 until a medical professional determines thatsufficient force has been applied to fasten the locking coupler assembly100 in place on the rod. According to some embodiments, the nut 106includes a locking mechanism for preventing the nut 106 from beingloosened inadvertently. The locking mechanism can be at least one of alock ring, a lock washer, a ratcheting surface, a lock nut, an adhesive,or the like. According to some embodiments, an installation/removal tool(not shown) is used to tighten the nut 106. By way of example, theinstallation/removal tool can be a screwdriver, nut driver, wrench, orthe like.

FIG. 7 illustrates a locking coupler assembly 100, in accordance withexample embodiments. As illustrated, the mating surface 114 of thecoupler 102 may have a smooth finish 700. According to some embodiments,the smooth finish 800 provides sufficient clamping force to an object,such as a rod, as to hinder movement or rotation of the locking couplerassembly 100 when it is fixably attached to the object. However, thesmooth finish 700 may still allow the rod (or other object) to slide inthe coupler 102.

FIG. 8 illustrates another embodiment of a locking coupler assembly 100.As illustrated, the mating surface 114 of the coupler 102 may have atextured finish 800. According to some embodiments, the textured finish800 is made at least one of ridges, abrasions, serrations, divots, orthe like to improve the clamping force applied by the extension arm 118when the locking coupler assembly 100 is tightened around an object,such as a rod.

FIGS. 9 and 10 illustrate use of locking coupler assemblies 100 a and100 b with a distractor tool 900, in accordance with exampleembodiments. The distractor tool 900 may be coupled to the lockingcoupler assemblies 100 a and 100 b, referred to individually as firstlocking coupler assembly 100 a and second locking coupler assembly 100b. As illustrated, the distractor tool 900 may include an adjustmentknob 902, a screw 904, a first arm 906, a second arm 908, and pivotingblocks 911. As illustrated, the screw 904 may extend from the adjustmentknob 902 through the pivoting blocks 911. While not shown, the pivotingblocks 911 may have a threaded through passage that engages the screw904. Alternatively, a ratcheting mechanism may be used in place ofthreading. The first arm 906 and the second arm 908 may each have aproximal end 914 and a distal end 916. The first arm 906 and the secondarm 908 may also be coupled at pivot 912. Accordingly, when proximalends 914 of the first arm 906 and second arm 908 are moved together, thedistal ends of the first arm 906 and the second arm 908 are moved apart.The proximal ends 914 may be secured to the pivoting blocks 911. Thedistal ends 916 may be secured to the locking coupled assemblies 100 aand 100 b. As illustrated, the first arm 906 and the second arm 908 maybe attached to the locking coupler assemblies 100 a and 100 b byconnecting to the attachment lug 124 of FIG. 1. For example, the firstarm 906 may be coupled to the first locking coupler assembly 100 a, andthe second arm 908 may be coupled to the second locking coupler assembly100 b. In operation, the adjustment knob 902 may be rotate to pull thepivoting blocks 110 and in turn the proximal ends 914 of the first arm906 and second arm 908 closer to one another. By moving the proximalends 914 closer to another, the distal ends 916 and, in turn, thelocking coupler assemblies 100 a and 100 b may be moved apart. Inoperation, the first arm 906 and the second arm 908 may pivot withrespect to the pivoting blocks 911, for example, to maintain the lockingcoupler assemblies 100 a and 100 b parallel when moved apart from oneanother.

Rod 910 is also illustrated on FIGS. 9 and 10. While not shown, the rod910 may be is affixed to the vertebrae of a spinal column using screwsor other fasteners to limit curvature and improve bone alignment.According to some embodiments, the rod 910 is fixably attached tovertebrae of the spine during a surgical procedure. As shown on FIG. 9,the locking coupler assemblies 100 a and 100 b may be placed onto therod 910 in an untightened state in accordance with example embodiments.FIG. 10 illustrates the locking coupler assemblies 110 a and 100 battached to the rod 910 after tightening in accordance with exampleembodiments. As a result, the distraction tool 900 may be used to pushapart (or pull together) the two locking coupler assemblies 100 and 100b. For example, the two locking coupler assemblies 100 a and 100 b maybe pushed apart or pulled together along the rod 910 depending onwhether the adjustment knob 904 of the distractor tool 900 is turnedclockwise or counter-clockwise.

Operation of locking coupler assemblies 100 a and 100 b in a spinalprocedure will now be described in more detail with respect to FIGS.11-14. As illustrated, rod 910 may be arranged in a segment of spinalcolumn 1100. Bone fasteners 1104 distributed along the spinal column1100 in corresponding vertebrae 1102. Suitable bone fasteners 1104 mayinclude, but are not limited to, pedicle screws or single or doublevertebral hooks with a fixation head for the rod 910. The rod 910 may bearranged in the spinal column 1100 to link the fasteners together.However, the rod 910 may not yet be fixed to one or more of the bonefasteners 1104 in a locked position. With reference to FIG. 11, thelocking coupler assemblies 100 a, 100 b may be installed onto the rod900, for example, as previously described. For example, the lockingcoupler assemblies 100 a, 100 b may be clamped onto a segment of the rod910. In some embodiments, the first locking coupler assembly 100 a mayhave a smooth finish 700 (e.g., shown on FIG. 7), thus allowing movementon the rod 910 when clamped thereon while the second locking couplerassembly 100 b may have a textured finish 800 (e.g., shown on FIG. 8),thus restricting movement along the rod 910 when clamped thereon. Onceinstalled on the rod 910, the distractor tool 900 may be operated toforce the locking coupler assemblies 100 a and 100 b apart. For example,the first locking coupler assembly 100 may be moved along the rod 910,as shown on FIG. 12), while the second locking coupler assembly 100 bremains stationary. As the locking coupler assemblies 100 a and 100 bare forced apart, movement of the corresponding bone fasteners 1104,and, thus, the vertebrae 1102 is induced, forcing adjacent vertebraemore into alignment by parallelism. When the first locking couplerassembly 100 a has been moved to a desired position along the rod 910,embodiments may then include locking the rod 910 in the bone fasteners110 in the segment of the rod 910 in which the locking couplerassemblies 100 a, 100 b were applied. The locking coupler assemblies 110a and 100 b may then be loosened and removed from the rod 910. Asillustrated by FIG. 13, embodiments may include attaching the lockingcoupler assemblies 100 a and 100 b to the rod 910 at another position inthe spinal column 1100. The distractor tool 900 may then be operated toforce the locking coupler assemblies 100 a and 100 b apart, as shown inFIG. 14. When the first locking coupler assembly 100 a has been moved toa desired position along the rod 910, embodiments may then includelocking the rod 910 in the bone fasteners 110 in the additional segmentof the rod 910 in which the locking coupler assemblies 100 a, 100 b weremoved.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. An apparatus for aligning vertebrae of a curvedspine, comprising: a coupler having a longitudinal axis, a proximal end,and a distal end, wherein the proximal end comprises first threads,wherein the distal end comprises a mating surface shaped to receive andsecure a rod attached to a plurality of bone screws that are alreadyinserted into vertebral bodies and that have already received the rod ina head sleeve of the inserted bone screws, and wherein the couplerapplies a compressive clamping force to the rod; a nut having secondthreads corresponding to the first threads, wherein the nut isattachable to the coupler proximal end; and a driver disposable at leastpartially around the coupler, wherein the driver applies a clampingforce to the coupler, and wherein the nut applies a linear force to thedriver when tightened in order to secure the rod to the coupler, acoupling element adapted to slidably secure the rod and to slide againstone of the inserted bone screws relative to the coupler secured to therod in order to adjust a distance between two inserted bone screws, theadjustment causing one inserted bone screw to slide along the rod. 2.The apparatus of claim 1, wherein the mating surface of the distal endof the coupler comprises a textured surface for gripping the rod.
 3. Theapparatus of claim 1, wherein the mating surface of the distal end ofthe coupler comprises a smooth surface for gripping the rod.
 4. Theapparatus of claim 1, wherein the coupler comprises a split beginning atthe distal end and extending a distance along the longitudinal axis topermit the driver to apply the clamping force by radially compressingthe distal end of the coupler.
 5. The apparatus of claim 1, wherein thespring like extension arm flexes radially towards the longitudinal axisof the coupler, and wherein the extension arm comprises portions of themating surface.
 6. The apparatus of claim 1, wherein the couplercomprises an angled shoulder at a central region of the coupler betweenthe proximate end of the distal end.
 7. The apparatus of claim 1,wherein the coupler comprises an angled outer surface, and wherein thedriver comprises an angled inner surface, and wherein the driverradially compresses the coupler.
 8. The apparatus of claim 1, whereinthe driver comprises an attachment lug and a forcing surface thatengages a corresponding shoulder on the coupler.
 9. A system foraligning vertebrae of a curved spine, comprising: a pair of lockingcoupler assemblies, wherein each of the locking coupler assembliescomprises: a coupler having a longitudinal axis, a proximal end, and adistal end, wherein the proximal end comprises first threads, whereinthe distal end comprises a mating surface shaped to receive and secure arod attached to a plurality of bone screws that are already insertedinto vertebral bodies and that have already received the rod in a headsleeve of the inserted bone screws, and wherein the coupler applies acompressive clamping force to the rod; a nut having second threadscorresponding to the first threads, wherein the nut is attachable to thecoupler proximal end; and a driver disposable at least partially aroundthe coupler, wherein the driver applies a clamping force to the coupler,and wherein the nut applies a linear force to the driver when tightened;and a distractor tool attachable to the locking coupler assemblies andoperable to move the locking coupler assemblies from one another, one ofthe locking coupler assemblies adapted to slide against one of theinserted bone screws relative to the other locking coupler assembly inorder to adjust a distance between two inserted bone screws, theadjustment causing one inserted bone screw to slide along the rod. 10.The system of claim 9, wherein the locking coupler assemblies comprisesa first locking coupler assembly and a second locking coupler assembly,wherein the distractor tool comprise a first arm attached to the nut ofthe first locking coupler assembly and a second arm attached to the nutof the second locking coupler assembly, wherein the first arm and thesecond arm are coupled at a pivot.
 11. The system of claim 10, whereinthe distractor tool further comprises an adjustment knob, pivotingblocks, and a screw that extends from the adjustment knob through thepivoting blocks, wherein rotation of the screws moves the pivotingblocks with respect to one another, and wherein the pivoting blocks arepivotably coupled to the first arm and the second arm.
 12. The system ofclaim 9, wherein the mating surface of the distal end of the couplercomprises a smooth surface for gripping the rod.
 13. The system of claim9, wherein the distal end comprises an extension arm that flexesradially towards the longitudinal axis of the coupler, and wherein theextension arm comprises portions of the mating surface.
 14. The systemof claim 9, wherein the coupler comprises an angled shoulder at acentral region of the coupler between the proximate end of the distalend.
 15. The system of claim 9, wherein the coupler comprises an angledouter surface, and wherein the driver comprises an angled inner surface,and wherein the driver radially compresses the coupler.
 16. The systemof claim 9, wherein the driver comprises an attachment lug that receivesan arm of the distraction tool and a forcing surface that engages acorresponding shoulder on the coupler.
 17. A method for aligningvertebrae of a curved spine, comprising: tightening a first lockingcoupler assembly onto a rod, wherein the rod is attached to a spinalcolumn by way of bone fasteners distributed along the spinal column,which have already been inserted into vertebral bodies and which havealready received the rod in a head sleeve of the inserted bonefasteners; tightening a second locking coupler assembly onto the rod;and forcing the first locking coupler assembly to slide against one ofthe bone fasteners relative to the second locking coupler assembly tocause one inserted bone fastener to slide along the rod such that one ormore of the bone fasteners are forced more into alignment with anadjacent one of the bone fasteners.
 18. The method of claim 17, whereinthe tightening comprises: inserting a mating surface of a coupler of thefirst locking coupler assembly onto the rod; and driving a driver of thelocking coupler assembly onto the coupler to clamp an extension arm ofthe coupler onto the rod.
 19. The method of claim 18, wherein the firstlocking coupler assembly and the second locking coupler assembly remainparallel while the first locking coupler assembly is moved along therod.